Post-heat treatment for polyvinylidene chloride-coated film

ABSTRACT

In the preparation of a laminate of polyvinylidene chloridecoated nylon film with polyethylene film, the polyvinylidene chloride-coated nylon film is heated to at least about 110* F. for a period of time prior to applying adhesive for the lamination. This post-heat treatment of the coated nylon affords a laminate with very low retained solvent level without the need for lengthy storage. The product is suitable for packaging food products.

United States Patent Wang 1 51 May 9,1972

[54] POST-HEAT TREATMENT FOR POLYVINYLIDENE CHLORIDE- COATED FILM [72]Inventor: James C. Wang, Kendall Park, NJ.

[73] Assignee: Allied Chemical Corporation, New York,

[22] Filed: Oct. 10, 1969 [21] Appl.No.: 865,335

52 u.s.c1 ..156/3l5,117/62,156/322, l61/227,l6l/247, 161/256 51 1111.01...C09j5/00 58 Field ofSearch ..156/322, 315; 117/62; 161/190, 161 227,247, 256

[56] References Cited UNITED STATES PATENTS 3,514,367 5/1970 James..161/2,6X

2,768,905 10/1956 Massey ..l56/322 X 3,370,972 2/1968 Nagel et al ..117/7 3,420,679 l/1969 Gifford et a1. ..99/l7l 3,518,103 6/1970 Visnovsky..161/l90X Primary Examiner-Carl D. Quarforth Assistant E.\'aminer-R. L.Tate Attorney.lonathan Plaut [5 7] ABSTRACT 2 Claims, 1 Drawing FigurePatented May 9, 1972 I I 3,661,677

INVENTOR JAMES C.WANG

POST-HEAT TREATMENT FOR POLYVINYLIDENE CHLORIDE-COATED FILM BACKGROUNDOF THE INVENTION This invention relates to a process for the productionof nylon-polyethylene film laminates, and in particular to theproduction of polyvinylidene chloride-coated nylonpolyethylene filmlaminates having a low level of retained solvent.

A general practice has developed in the packaging industry of utilizingfilm laminates where a single film would not provide all characteristicsneeded in a particular packaging application. For instancenylon-polyethylene film laminates are often used wherein the nylon filmaffords a laminate which is readily thermoformable, ductile and strong,and the polyethylene film gives the laminate capacity for rapid heatsealing. These laminates have been modified in certain instances where alow permeability to oxygen was required, such as in packaging ofluncheon meats, cheeses, etc. For this purpose, a coating ofpolyvinylidene chloride has been applied to the nylon film prior tolamination, which coating served as an effective oxygen barrier.Typically, these laminates are produced by coating the nylon withpolyvinylidene chloride, then applying a solution of adhesive to thepolyvinylidene chloride surface, evaporating the solvent from theapplied adhesive, and laminating the coated nylon with the polyethylenefilm by passing the two through nip rolls. The various techniques whichare commonly utilized in applying the coatings to the film and inlaminating the films are disclosed in detail in "Fundamentals toConsider in Selecting Coating Methods" Paper, Film and Foil Converter,February through July, 1963.

When the laminate is to be used in food packaging and other applicationsin which retained solvent might be harmful, it is necessary to reducethe level of solvent retained from application of the adhesive solutionto a low level, for example, to less than about 100 parts per million(p.p.m.). For this purpose, after application of the adhesive, thepolyvinylidene chloride-coated film is passed through a drying oven, orsubjected to some other drying means. It has been found, however, thatunreasonably long residence times are required to reduce the solventcontent to an acceptable level, in contrast to short times required whenthe polyvinylidene coating is not present. In fact, it is oftennecessary to store the coated nylon for a substantial length of time inorder to reduce the solvent level to less than 100 p.p.m.

SUMMARY OF THE INVENTION It has nowbeen found that retained solventlevels of less than 100 p.p.m. can be readily attained innylon-polyethylene film laminates having an intermediate layer ofpolyvinylidene chloride by post-heat treatment of the polyvinylidenechloride-coated nylon film prior to applying adhesive thereto. Accordingto the instant invention, laminates with low retained solvent levels areproduced by applying a polyvinylidene chloride coating to the surface ofnylon film and heating the coated nylon film at a temperature of atleast about l 10 F. for a time up to about 6 hours, which heating issufficient to render the polyvinylidene chloride coating substantiallyfully crystalline. A solvent-based adhesive is then applied to thecrystalline polyvinylidene chloride coating which is then dried toremove solvent therefrom and provide a retained solvent level less thanabout 100 p.p.m. in the laminate product. Polyethylene film is thenlaminated to the polyvinylidene chloride coating of the nylon film toafford the finished product.

BRIEF DESCRIPTION OF DRAWING For a better understanding of the instantinvention, reference is made to the accompanying drawing wherein isprovided a schematic diagram of the instant process.

DETAILED DESCRIPTION OF THE INVENTION The process of the instantinvention for the production of polyvinylidene chloride-coated nylonpolyethylene film laminates involves the addition of a post-heattreatment step to the lamination process as conducted heretofore. Withreference to the accompanying drawing, primer is applied to the surfaceof advancing nylon film 10 by means of gravure roll 11 and coated film10 is passed through drying oven 12, following which a coating ofpolyvinylidene chloride is applied over the primer by means of pick-uproll 13 and metering rod 14. The resulting coated nylon film is thensubjected to the novel post-heat treatment step by passing the filmthrough drying oven 15, after which treatment solvent-based adhesive isapplied to the polyvinylidene chloride coating by means of gravure roll16. Depending upon the temperature and residence time in drying oven 15,it may be desirable to interrupt the process, as indicated at 17, andstore coated film 10 for a short time at elevated temperature prior toapplying the adhesive. This optional storage at elevated temperatureconstitutes a continuation of the post-heat treatment step initiated indrying oven 15. After applying the adhesive, the solvent is then removedby passing the film through drying oven 18. As coated nylon film 10exits oven 18 it is laminated to polyethylene film 19 by passing the twofilms between nip roll 20 and binding roll 21. The resulting filmlaminate 22 is collected on wind-up roll 23.

The amount of polyvinylidene chloride applied to the nylon film isdetennined by the effectiveness of the oxygen barrier desired.Typically, a permeability to oxygen in the range of less than about 0.5cc./atm./24 hours/ in is desired, for which purpose between about 3.5and 4.5 pounds of polyvinylidene chloride will be applied per ream ofnylon film (3,000 ft). Of course, greater or lesser amounts may beapplied consistently with the instant invention, depending upon thedesired specifications of the finished laminate product. The

coating can be applied by any of the methods generally familiar to thoseskilled in the art, including those methods outlined in the referencescited hereinbefore. Primer coat may be applied to the film prior to thepolyvinylidene chloride coat; this feature is not essential forsuccessful production of laminates but has often been utilizedheretofore to obtain better adhesion between the nylon film andpolyvinylidene chloride.

After application of the polyvinylidene chloride coat to the nylon film,the coated film is subjected to a novel heat treatment step, rather thanto direct application of adhesive as has been the practice heretofore.It has been found that laminates prepared according to the instantimproved process have retained solvent levels as low as 1 percent of thelevels obtained when the heat treatment step is not employed. A minimumtemperature of F. is preferred for this heat treatment step. Inasmuch asthis treatment must be sufficient to render the polyvinylidene chloridesubstantially fully crystalline, the time of heating will vary with thetemperature employed. When the film is heated at 110 F., it has beenfound that 6 hours of heating are sufficient to afford the desiredcrystallinity, and as higher temperatures are employed, the requiredtime will correspondingly diminish. It is preferred to heat the film ata temperature within the range of about 110 F.-220 F., more preferablywithin the range of about 110 F.-l40 F., with the heating time rangingup to about 6 hours and being sufficient to render the polyvinylidenechloride film substantially fully crystalline at the particulartemperature employed. Whether any specific set of time and temperatureconditions are sufficient for this purpose can be readily determinedbeforehand by subjecting the coated nylon to such conditions and thendetermining the degree of crystallinity, for example, by means of suchstandard techniques as attenuated total reflectance. Inasmuch as thethickness of the polyvinylidene chloride coating on the nylon film, itis desirable to re-evaluate the adequacy of the heat treatment when thethickness of the coating is varied.

The heat treatment can be accomplished by any of a variety of standardmethods familiar to those with skill in the art, for example, by meansof hot air, radiant sources, heated rolls, etc. Depending upon therequired residence time in the heat treatment, it may be advantageous tosubject the film to it while it is advancing to adhesive-applicationstage, or it may be advantageous to collect the film temporarily forthis purpose. I

After the heat treatment, solvent-based adhesive is applied to thepolyvinylidene chloride coating of the film. Any of the standardadhesive compositions can be utilized for this purpose, typically, forexample, a polyurethane composition containing -20 percent solids incommon solvents such as ethyl acetate, methyl ethyl ketone, toluene andmixtures thereof. These solvent-based adhesives are commerciallyavailable materials. They are applied to the advancing film by gravureroll, metering rod, or other coating techniques commonly employed, afterwhich application the solvent is removed by evaporation. Heretofore,without the heat treatment step of the instant invention, great efforthas been required to remove substantially all the solvent so that thefinished laminates could be used in conjunction with foods and othersolvent-sensitive materials. However, with the instant invention,retained solvent levels lower than 100 ppm, and often less than 10 ppm,are readily obtained by passing the film through a drying oven.

The following example is provided to illustrate the instant inventionmore fully. It is provided for illustrative purposes only and should notbe construed as limiting the invention, which is defined in the appendedclaims. It will be clear that various modifications and variations canbe made from the procedure of the example without departing from thescope of the invention. In the example, all temperatures are expressedin Fahrenheit degrees.

EXAMPLE To anadvancing sheet of 0.9 mil nylon 6 film is applied by meansof a gravure roll a polyurethane primer coat at the rate of 0.2 poundsof primer per ream (3,000 ft*). After the coated film is dried,polyvinylidene chloride is then applied over the primer coat by means-of a metering rod, at the rate of 4 pounds per ream, which issufiicient to provide the finished laminate with an oxygen permeabilityless than 0.5 cc./atm./24 hours/100 in. Following application of thepolyvinylidene chloride, the nylon film is heat treated by maintainingit in a 1 10 F. oven for 6 hours, thereby rendering the polyvinylidenechloride coating substantially fully crystalline. Polyurethane adhesive(20 percent solids in ethyl acetate) is applied over the polyvinylidenechloride coating via gravure roll at the rate of 1.2 pounds per ream,following which application the film is passed into a drying ovenwherein a carrier belt carries the adhesive bearing surface of the filmbeneath a series of hot (greater than 200 F.) air nozzles to evaporatethe solvent. The nylon film is then laminated to a 1.5 mil polyethylenefilm by passing the two between a nip roll and a heated binding roll toafi'ord the finished material which is collected on a windup roll.

This procedure is repeated with additional samples of film, wherein theconditions of the heat treatment are varied. Approximate retainedsolvent levels for each set of conditions are set forth below.

Approximate Run No. Heat Treatment Retained Solvent Temp.(F.)Time(l'lrs.) Level l 3.5 350-400 ppm 2 1 10 3.5 250-300 ppm 3 6.0 3 ppm4 3.5 3 ppm 5 140 6.0 3 ppm What is claimed is:

1. In the process for production of nylon-polyethylene film laminatehaving an intermediate layer of polyvinylidene chloride, which processincludes the steps of applying a polyvinylidene chloride coating tonylon film, applying a solvent-based adhesive to the surface of saidpolyvinylidene chloride coating, drying said coated nylon film tosubstantially remove said solvent therefrom, and laminating polyethylenefilm to the polyvinylidene chloride coating of said nylon film, theimprovement which comprises:

heating said coated nylon film, prior to applying said so]- vent-basedadhesive thereto, at a temperature between about 110 F. and 140 F. for atime up to about 6 hours, said heating being sufficient to render saidpolyvinylidene chloride coating substantially fully crystalline; and

drying solventfrom said coated nylon film after applying thereto saidsolvent-based adhesive to provide a retained solvent level less thanabout 100 p.p.m. in the laminate product.

2. The improved process of claim 1 wherein the retained solvent level insaid laminate is less than about 50 p.p.m.

2. The improved process of claim 1 wherein the retained solvent level insaid laminate is less than about 50 p.p.m.